VSIPL16 — Block reference-counting only described in Dense

It is clear from the existence of the increment_count and
decrement_count functions in the block requirements that the expectation
is that the reference-counting should apply to all VSIPL++ blocks.

However, there is no text at all that states this, aside from those two
function signatures.

There is no definition of "use count" anywhere. (See also my previous
issue noting that we need to explicitly state that the block constructor
initializes it to one.)

The only definition of the increment_count and decrement_count is in the
section describing the Dense block, although those definitions ought to
apply to all blocks.

This needs to be rearranged.